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The release of a video showing the murder of the British aid worker David Haines - and the threat to behead another British hostage - is expected to increase pressure on David Cameron to take the pursuit of the Islamic State (Isis) into Syria.
Mr Cameron was meeting with senior security aides on Sunday morning after the emergence of the footage on Saturday night, and will make a further statement later today.
But senior military figures said it was vital for Mr Cameron not to be “cowed” by the murder, and an explicit claim in the Isis video that he was personally responsible for Mr Haines’ death.
Colonel Richard Kemp, the former commander of British forces in Afghanistan, said: “Having worked in British Government on hostage rescue I know how much effort has gone in to saving David Haines. Now is the time for retribution.”
Former head of the Army Lord Dannatt said the UK should respond by playing its role in the assault against Isis promised by US President Barack Obama.
“What we absolutely need to do is not be cowed in any way by yet another foul murder of a hostage,” he told Sky News.
“But to develop the strategy into a sensible military campaign in coalition with regional players such as Saudi Arabia, Jordan and other countries in the area.
“We can support them to confront, attack and defeat the Islamic State jihadi fighters ... and make sure this cancer is removed from the region before it spreads more widely.”
Sky News footage showed David Cameron arriving back at Downing Street after the footage emerged late on Saturday night (Sky News) In separate statements, both Mr Cameron and Mr Obama condemned the killing and vowed to bring the perpetrators to justice.
The Prime Minister said: “This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.
“My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal.
“We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”
Foreign Secretary Philip Hammond (R) arrives for a meeting of the emergency response committee, Cobra, in Downing Street on Sunday morning (AFP) And Mr Obama, who recently announced moves to target Isis in Syria, said: “The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group Isil (Isis).
“Our hearts go out to the family of Mr Haines and to the people of the United Kingdom. The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve.
“We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world.”
Mr Cameron returned to Downing Street shortly after midnight and is chairing a meeting of the Government's emergency Cobra committee this morning.
Show all 74 1 /74In pictures: The rise of Isis In pictures: The rise of Isis Isis fighters Fighters of the Islamic State wave the group's flag from a damaged display of a government fighter jet following the battle for the Tabqa air base, in Raqqa, Syria
AP
In pictures: The rise of Isis Isis fighters Fighters from Islamic State group sit on their tank during a parade in Raqqa, Syria
AP
In pictures: The rise of Isis Isis fighters Fighters from the Islamic State group pray at the Tabqa air base after capturing it from the Syrian government in Raqqa, Syria
AP
In pictures: The rise of Isis Isis fighters Fighters from extremist Islamic State group parade in Raqqa, Syria
AP
In pictures: The rise of Isis Isis kidnapping A video uploaded to social networks shows men in underwear being marched barefoot along a desert road before being allegedly executed by Isis
Getty Images
In pictures: The rise of Isis Isis kidnapping Haruna Yukawa after his capture by Isis
In pictures: The rise of Isis Isis kidnapping Khalinda Sharaf Ajour, a Yazidi, says two of her daughters were captured by Isis militants
Washington Post
In pictures: The rise of Isis Isis fighters Spokesperson for Isis
Vice News via Youtube
In pictures: The rise of Isis A pro-Isis leaflet A pro-Isis leaflet handed out on Oxford Street In London
Ghaffar Hussain
In pictures: The rise of Isis Isis fighters Isis Jihadists burn their passports
In pictures: The rise of Isis Isis controls Syrian Aid A man collecting aid administered by Isis in Syria
In pictures: The rise of Isis Isis controls Syrian Aid A woman collecting aid administered by Isis in Syria
In pictures: The rise of Isis Isis controls Syrian Aid Local civilians queue for aid administered by Isis. Since it declared a caliphate the group has increasingly been delivering services such as healthcare, and distributing aid and free fuel
In pictures: The rise of Isis Iraq crisis Iraqi security forces detain men suspected of being militants of the Isis group in Diyala province
In pictures: The rise of Isis Iraq crisis Mourners carry the coffin of a Shi'ite volunteer from the brigades of peace, who joined the Iraqi army and was killed during clashes with militants of the Isis group in Samarra, during his funeral in Najaf
In pictures: The rise of Isis Iraqi refugees An Iraqi Shiite Turkmen family fleeing the violence in the Iraqi city of Tal Afar, west of Mosul, arrives at a refugee camp on the outskirts of Arbil, in Iraq's Kurdistan region
In pictures: The rise of Isis Isis leader Abu Bakr al-Baghdadi A photograph made from a video by the jihadist affiliated group Furqan Media via their twitter account allegedly showing Isis leader Abu Bakr al-Baghdadi delivering a sermon during Friday prayers at a mosque in Mosul. Abu Bakr al-Baghdadi declared an Islamist caliphate in the territory under the group's control in Iraq and Syria
In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq Shiite's Al-Qubba Husseiniya mosque explodes in Mosul
In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq Smoke and debris go up in the air as Shiite's Al-Qubba Husseiniya mosque explodes in Mosul. Images posted online show that Islamic extremists have destroyed at least 10 ancient shrines and Shiite mosques in territory - the city of Mosul and the town of Tal Afar - they have seized in northern Iraq in recent weeks
In pictures: The rise of Isis Islamic extremists destroying mosques in Iraq A bulldozer destroys Sunni's Ahmed al-Rifai shrine and tomb in Mahlabiya district outside of Tal Afar
In pictures: The rise of Isis Iraq crisis Iraqi security forces celebrate after clashes with followers of Shiite cleric Mahmoud al-Sarkhi, in front of his home in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad
In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad
In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi at his home after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad
In pictures: The rise of Isis Iraq crisis Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad
In pictures: The rise of Isis Iraq crisis A vehicle burns in front of a home of a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad
In pictures: The rise of Isis Iraqi refugees An Iraqi woman holds her exhausted son as over 1000 Iraqis who have fled fighting in and around the city of Mosul and Tal Afar wait at a Kurdish checkpoint in the hopes of entering a temporary displacement camp in Khazair
In pictures: The rise of Isis Iraqi refugees Displaced Iraqi women hold pots as they queue to receive food during the first day of the Islamic holy month of Ramadan, at an encampment for displaced Iraqis who fled from Mosul and other towns, in the Khazer area outside Irbil, north Iraq
In pictures: The rise of Isis Isis fighters in Syria A militant Islamist fighter waving a flag, cheers as he takes part in a military parade along the streets of Syria's northern Raqqa. The fighters held the parade to celebrate their declaration of an Islamic "caliphate" after the group captured territory in neighbouring Iraq
In pictures: The rise of Isis Isis fighters in Syria Isis fighters wave flags as they take part in a military parade along the streets of Syria's northern Raqqa province
Reuters
In pictures: The rise of Isis Isis fighters in Syria Isis fighters travel in a vehicle as they take part in a military parade along the streets of Syria's northern Raqqa province
In pictures: The rise of Isis Isis fighters in Syria Fighters from the Isis group during a parade with a missile in Raqqa, Syria. Militants from an al-Qaida splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border
In pictures: The rise of Isis Isis fighters in Syria Isis fighters during a parade in Raqqa, Syria
In pictures: The rise of Isis Isis fighters in Syria Fighters from the Isis group during a parade in Raqqa, Syria. Militants from the splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border
In pictures: The rise of Isis Isis fighters in Syria Isis fighters hold a military parade in their stronghold in northeastern Syria
In pictures: The rise of Isis Isis fighters in Syria Isis fighters during a parade in Raqqa, Syria
In pictures: The rise of Isis Isis fighters in Syria A member loyal to the Isis waves an Isis flag in Raqqa
In pictures: The rise of Isis Iraq crisis Iraqi anti-government gunmen from Sunni tribes in the western Anbar province march during a protest in Ramadi, west of Baghdad. The United Nations warned that Iraq is at a "crossroads" and appealed for restraint, as a bloody four-day wave of violence killed 195 people. The violence is the deadliest so far linked to demonstrations that broke out in Sunni areas of the Shiite-majority country more than four months ago, raising fears of a return to all-out sectarian conflict
In pictures: The rise of Isis Iraq crisis Iraqi security forces hold up a flag of the Isis group they captured during an operation to regain control of Dallah Abbas north of Baqouba, the capital of Iraq's Diyala province, 35 miles (60 kilometers) northeast of Baghdad
In pictures: The rise of Isis Isis fighters in Iraq Isis fighters parade in the northern city of Mosul
In pictures: The rise of Isis Iraq crisis Volunteers, who have joined the Iraqi army to fight against the predominantly Sunni militants from the radical Isis group, demonstrate their skills during a graduation ceremony after completing their field training in Najaf
In pictures: The rise of Isis Iraq crisis Kurdish Peshmerga troops fire a cannon during clashes with militants of the Isis group in Jalawla, Diyala province
In pictures: The rise of Isis Lieutenant General Qassem Atta speaks during a press conference Iraqi Prime Minister's security spokesman, Lieutenant General Qassem Atta speaks during a press conference about the latest military development in Iraq, in the capital Baghdad. Iraqi forces pressed a campaign to retake militant-held Tikrit, clashing with jihadist-led Sunni militants nearby and pounding positions inside the city with air strikes in their biggest counter-offensive so far
In pictures: The rise of Isis A police station building destroyed by Isis fighters An exterior view of a police station building destroyed by gunmen in Mosul city, northern Iraq. Iraq's new parliament is expected to convene to start the process of setting up a new government, despite deepening political rifts and an ongoing Islamist-led insurgency. Iraqi President Jalal Talabani issued a decree inviting the new House of Representatives to meet and form a new government
In pictures: The rise of Isis Isis fighters in Iraq Smoke billows from an area controlled by the Isis between the Iraqi towns of Naojul and Tuz Khurmatu, both located north of the capital Baghdad, as Iraqi Kurdish Peshmerga forces take part in an operation to repel the Sunni militants
In pictures: The rise of Isis Iraqi refugees An elderly Iraqi woman is helped into a temporary displacement camp for Iraqis caught-up in the fighting in and around the city of Mosul in Khazair
In pictures: The rise of Isis Iraqi refugees An Iraqi Christian woman fleeing the violence in the village of Qaraqush, about 30 kms east of the northern province of Nineveh, cries upon her arrival at a community center in the Kurdish city of Arbil in Iraq's autonomous Kurdistan region
In pictures: The rise of Isis Iraqi refugees An Iraqi woman, who fled with her family from the northern city of Mosul, prays with a copy of the Quran
AP
In pictures: The rise of Isis Isis fighters in Iraq The body of an Isis militant killed during clashes with Iraqi security forces on the outskirts of the city of Samarra
Reuters
In pictures: The rise of Isis Iraq crisis Iraqi civilians inspect the damage at a market after an air strike by the Iraqi army in central Mosul
EPA
In pictures: The rise of Isis Iraq crisis Members of the Al-Abbas brigades, who volunteered to protect the Shiite Muslim holy sites in Karbala against Sunni militants fighting the Baghdad government, parade in the streets of the city
AP
In pictures: The rise of Isis Iraq crisis Shia tribesmen gather in Baghdad to take up arms against Sunni insurgents marching on the capital. Thousands have volunteered to bolster defences
AFP/Getty
In pictures: The rise of Isis Iraq crisis A van carrying volunteers joining Iraqi security forces against Jihadist militants. Prime Minister Nuri al-Maliki announced the Iraqi government would arm and equip civilians who volunteered to fight
AFP/Getty
In pictures: The rise of Isis Iraq Fighters of the Isis group parade in a commandeered Iraqi security forces armored vehicle down a main road at the northern city of Mosul
In pictures: The rise of Isis Iraq An Islamist fighter, identified as Abu Muthanna al-Yemeni from Britain (R), speaks in this still image taken undated video shot at an unknown location and uploaded to a social media website. Five Islamist fighters identified as Australian and British nationals have called on Muslims to join the wars in Syria and Iraq, in the new video released by the Isis
In pictures: The rise of Isis Iraq Al-Qa’ida inspired militants stand with captured Iraqi Army Humvee at a checkpoint belonging to Iraqi Army outside Beiji refinery some 250 kilometers (155 miles) north of Baghdad. The fighting at Beiji comes as Iraq has asked the U.S. for airstrikes targeting the militants from the Isis group. While U.S. President Barack Obama has not fully ruled out the possibility of launching airstrikes, such action is not imminent in part because intelligence agencies have been unable to identify clear targets on the ground, officials said
In pictures: The rise of Isis Iraq Militants attacked Iraq's main oil refinein Baiji as they pressed an offensive that has seen them capture swathes of territory, a manager and a refinery employee said
In pictures: The rise of Isis Iraq Militants from the Isis group parading with their weapons in the northern city of Baiji in the in Salaheddin province
In pictures: The rise of Isis Iraq A smoke rises after an attack by Isis militants on the country's largest oil refinery in Beiji, some 250 kilometers (155 miles) north of the capital, Baghdad. Iraqi security forces battled insurgents targeting the country's main oil refinery and said they regained partial control of a city near the Syrian border, trying to blunt an offensive by Sunni militants who diplomats fear may have also seized some 100 foreign workers
In pictures: The rise of Isis Iraq Militants of the Isis group stand next to captured vehicles left behind by Iraqi security forces at an unknown location in the Salaheddin province. For militant groups, the fight over public perception can be even more important than actual combat, turning military losses into propaganda victories and battlefield successes into powerful tools to build support for the cause
In pictures: The rise of Isis Iraq An injured fighter (C) from the Isis group after a battle with Iraqi soldiers at an undisclosed location near the border between Syria and Iraq
In pictures: The rise of Isis Iraq Fighters from the Isis aiming at advancing Iraqi troops at an undisclosed location near the border between Syria and Iraq
In pictures: The rise of Isis Iraq Fighters from the Isis group taking position at an undisclosed location near the border between Syria and Iraq
In pictures: The rise of Isis Iraq Fighters from the Isis group inspecting vehicles of the Iraqi army after they were seized at an undisclosed location near the border between Syria and Iraq
In pictures: The rise of Isis Iraq One Iraqi captive, a corporal, is reluctant to say the slogan, and has to be shouted at repeatedly before he obeys
Sky News
In pictures: The rise of Isis Iraq Iraqi captives held by the extremists
Sky News
In pictures: The rise of Isis Iraq Iraqi captives held by the extremists
Sky News
In pictures: The rise of Isis Iraq Militants of the Isis group force captured Iraqi security forces members to the transport
In pictures: The rise of Isis Iraq Militants of the Isis group transporting dozens of captured Iraqi security forces members to an unknown location in the Salaheddin province ahead of executing them
In pictures: The rise of Isis Iraq A major offensive spearheaded by Isis but also involving supporters of executed dictator Saddam Hussein has overrun all of one province and chunks of three others
In pictures: The rise of Isis Iraq Militants of the Isis group executing dozens of captured Iraqi security forces members at an unknown location in the Salaheddin province
In pictures: The rise of Isis Iraq Isis militants taking position at a Iraqi border post on the Syrian-Iraqi border between the Iraqi Nineveh province and the Syrian town of Al-Hasakah
In pictures: The rise of Isis Iraq Isis rebels show their flag after seizing an army post
AFP/Getty Images
In pictures: The rise of Isis Iraq Isis militants waving an Islamist flag after the seizure of an Iraqi army checkpoint in Salahuddin
Getty Images
In pictures: The rise of Isis Iraq Demonstrators chant slogans as they carry al-Qa’ida flags in front of the provincial government headquarters in Mosul, 225 miles (360 kilometers) northwest of Baghdad. In the week since it captured Iraq's second-largest city, Mosul, a Muslim extremist group has tried to win over residents and has stopped short of widely enforcing its strict brand of Islamic law, residents say. Churches remain unharmed and street cleaners are back at work
The news came hours after the family of Mr Haines issued a plea to his captors to contact them.
The video begins with an interview clip of the Prime Minister and then features Mr Haines dressed in orange overalls and kneeling down in front of a man holding a knife in what appears to be a desert location.
The victim looks into the camera and says: “My name is David Cawthorne Haines. I would like to declare that I hold you David Cameron entirely responsible for my execution.
Barack Obama vows to target the Islamic State with air strikes 'wherever they exist' (Saul Loeb-Pool/Getty Images) “You entered voluntarily into a coalition with the United States against the Islamic State just as your predecessor Tony Blair did, following a trend against our British prime ministers who can't find the courage to say no to the Americans.
“Unfortunately it is we the British public that in the end will pay the price for our Parliament's selfish decisions.”
The militant, who appears to have a British accent, then says: “This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State.”
The video then shows the man take a knife to Mr Haines' throat, before the victim's dead body is shown.
The militant adds: “If you, Cameron, persist in fighting the Islamic State then you like your master Obama will have the blood of your people on your hands.”
